It has already been proposed to use amino resins, such as urea-formaldehyde anionic resins, for the treatment of paper in order to improve the resistance thereof to moisture, i.e. the ability of paper to withstand humid atmospheres. Anionic urea-formaldehyde resins are used in the presence of alum for incorporation in a paper past or slurry, in the production of unbleached paper and at an acid pH. The past is then applied to the usual supporting surface to form the paper web which is dried and subjected to heat to cure the resin.
It is also possible in a generally similar manner to use a cationic urea-formaldehyde resin at an acid pH for substantially all kinds of paper pastes or slurries. Among the latter type of resins is cationic melamine resins which are also used at an acid pH.
The disadvantage of operating with amino resins at acid pHs is that there always are difficulties with corrosion of the apparatus and, frequently, the formation of free formaldehyde which is a danger to personnel and an atmospheric pollutant. Thus the difficulties with amino resins operably only under acid conditions have limited their widespread successful application.
It was also known to use polyamine-epichlorohydrin and polyaminde-epichlorohydrin condensed resins in a neutral or alkaline medium generally at a pH between 6 and 9, to obtain a significant improvement in the resistance of paper to moisture without the drawbacks of the systems described earlier.
However, polyamide-epichlorohydrin resins and like condensates have been found to be disadvantageous on other grounds. For example, they must be stored in concentrations which are relatively weak (of the order of 10 to 20%) to avoid gelatin with time. Since the products have a relatively poor shelf life and cannot be stored adequately in high concentrations, their use is problematical. Furthermore, some of the products have been found to have a tendency to foam and thus must be used with great care.